1. Field of the Invention
This invention is in the field of methods of controlling processes to produce desired products at desired rates and at minimum cost, and, more particularly, to a process controlled by a computer which substantially continuously determines the run/idle status of each of the process components to produce the optimum state of the process at a given time and controls the process to substantially operate in said optimum state.
2. Description of the Prior Art
The use of a digital computer to control complex processes of a process plant or of a portion of such a plant is well known in the process control art, including the use of computers in optimizing process operations. This, in practice, requires the computer to calculate the optimal set points, desired values for process variables, at which to run the process and/or the components of the process. In practice, the computer calculates the set points for the components of a process and/or the components thereof and then applies the set points to control loops, which can exist either in the computer or externally of the computer in individual controllers.
Processes in industrial plants are subject to disturbances which require compensating action in the form of adjustments to the operating variables, including the starting up of idle equipment or the shutting down of running equipment if the disturbance is large enough. Such disturbances can be the result of the accumulation of gradual changes over a substantial period of time. Examples of such disturbances include changes in feedstock availability or cost; fuel, steam, electrical power or other energy source availability or cost; demand or prices of products of the process; condition of processing equipment and machinery; ambient conditions; and others. Compensating operational actions include adjustments to equipment output rates; energy sources and rates; operating pressure, temperature and other process variables; feedstock types and rates; catalyst addition and removal rates; residence times; idle or running status of equipment or process units; and others. For many industrial processes, a number of different operational adjustments can be made. The effect of a given adjustment is not confined to one well-defined result, but rather produces a range of results, some larger and some smaller, throughout the process, due to the interactions of many parts of the process with each other. These can be caused by the sharing of feedstock and energy streams and product output capabilities. Other causes are the fact that the output of one part of the process is the input to another. For a large class of industrial processes, there are numerous products, each with a different value, and possibly, with a specified minimum or a maximum rate of production. There are often numerous feedstocks and energy sources, each with a different cost and possibly with a specified maximum or minimum availability. Often there are numerous alternate pieces of equipment, process components, and processing steps which may be selected, each with different operating characteristics and availability. These process characteristics make it difficult to manually keep the operating variables adjusted to maximize profit or minimize cost of operations.
On-line, closed-loop optimization of continous operating variables has been accomplished in the past, but not with the simultaneous real-time adjustment of idle and run statuses. On-line, real-time optimization techniques have been limited to continuous operating variables. In types of processes with a number of process units or components, only some of which need to be running under circumstances which prevail at least part of the time, prior art on-line, real-time techniques could not determine the optimal choice of which process component or unit should be put in a run status and which in an idle status at any particular point in time. This limitation is the result of the discontinous nature of the transition from idle to run status or vice-versa which is characteristic of most process components.
In complex industrial processes, such as that of an industrial power plant for a petro-chemical plant, for example, the plant will typically have several steam boilers for producing steam, turbines for driving electric generators to produce electrical power, feedwater pumps for the boilers, etc. The inputs to the plant will typically be several different types of fuel and electrical energy purchased from an electric utility. The output of the process to users could be steam at various pressures, electricity, compressed air, chilled water, etc. The plant produces or acquires products, utilities, for example, at rates the users of the output of the plant require.
Optimizing on a real-time basis the process of supplying the desired products, utilities in the example, to meet demands at minimum cost, particularly deciding when to change the status of process components from run to idle, or vice versa, is a function that heretofore process control computers have not been able to perform on a real-time basis.